In most belt drive applications the ability to maintain proper belt tension is important to ensure power transmission without slippage of the belt. The lowest tension span in a belt drive is commonly referred to as the slack side span. Tensioners are traditionally positioned on the slack side span of a belt drive and are tasked with maintaining the proper minimum belt tension in this span. Using the belt rotation direction as a guide, this span is the span located just after the power providing pulley or crankshaft in this case. For instance, as the crankshaft rotates, the slack side span will be the span where the belt has just left the crankshaft pulley and the tight side span will be the span approaching the crankshaft pulley.
Belt alternator starter (BAS) systems utilize an alternator that also functions as motor. This is sometimes referred to as a motor-generator. The operation of the BAS system is such that when the engine is running, the alternator primarily behaves in a traditional manner and the belt is loaded normally with the power being provided by the engine crankshaft pulley and loaded by the alternator. In BAS systems the drive is typically arranged to position the alternator as the next accessory after the belt passes over the crankshaft. In this arrangement, the belt tensioner should be located between the crankshaft pulley and the alternator. The tensioner is located just before the alternator using the belt rotation direction as a guide.
BAS systems bring a unique problem to the belt drive. The alternator acts as both a load on the belt drive and a power provider for the belt drive. The BAS system alternator is used to start the engine and the alternator is used to provide power to the engine. In start instances, the alternator pulley becomes a power provider for the drive. This typically transforms the location of the slack span in the drive to the span following the alternator pulley. Additionally, the tight side span is now the span between the alternator and the crankshaft. Since a traditional tensioner is designed to simply maintain a minimum level of slack side tension, the now high tension in the belt at the tensioner location causes extreme movement of the tensioner. Additionally, this situation creates the need for a second tensioner in a location on the new slack side span.
The traditional approach to solving this problem is to create a belt drive with two tensioners. This second tensioner is typically a tensioner with high resistance to movement away from the belt. The second tensioner is often an expensive hydraulic tensioner. This two tensioner arrangement also requires an excessively long belt to accommodate the multiple tensioners in the drive. This often results in an expensive solution.
Representative of the art is U.S. Pat. No. 7,494,434 which discloses an accessory drive for an engine with a belt driven starter generator adapted for driving and being driven by the engine. In an exemplary embodiment, the drive includes a first engine drive pulley and a second starter drive pulley. A drive belt engages the drive pulleys for driving either pulley from the other. A dual belt tensioner made as a preassembled unit has a carrier with a central pivot mounted to the engine and first and second carrier arms extending radially from the central pivot. A first tensioner mounted on the first arm carries a first tensioner pulley biased against a first belt run adjacent the second drive pulley that is slack during engine starting. A second tensioner pulley carried on the second arm is biased against a second belt run adjacent the second drive pulley that is taut during engine starting A hydraulic strut connected to the second arm, and preferably included in the preassembled unit, provides moderate biasing for the second tensioner pulley during normal engine operation and velocity sensitive resistance, to increased belt forces, that limits reactive movement of the second tensioner pulley during engine starting and transient engine operation.
What is needed is a tensioner having a first pivot arm and a second pivot arm mounted to a base, a flexible member trained between the first pivot arm and the second pivot arm so the pivot arms move in a coordinated manner, and a tensioner assembly mounted to the base engaging the flexible member. The present invention meets this need.